Quads acting like triangles?
Basically, quads are low-key made of triangles.

Quads have no magical ability to stay "flat" so if you move a vertex out of its original plane, it can't be a "real" quad anymore and so blender shows you the ugly truth: a quad is two triangles stuck together.

Generally you want to build models so the quads don't get bent much, but they often will a little, and that's ok as long as the result doesn't look bad.